This invention relates to engine oils useful in internal combustion engines and more particularly to engine oils having good antiwear and viscometric properties as well as other desirable properties including resistance to oxidation under conditions of high temperature, high speed and high load. The preferred engines oils of this type are synthetic oils but the advantages of the invention may be extended to oils containing base stocks of mineral origin.
Multi-grade engine oils, derived from a combination of low viscosity basestocks and high molecular weight thickeners, viscosity index improvers, and other components have been used for a long time. Synthetic engine oils based on polyalphaolefins (PAOs) have been shown to demonstrate performance benefits together with cost effectiveness in automotive and other engine applications. In these synthetic oils, as with conventional oils of mineral origin, the viscosity-temperature relationship of the oil is one of the critical criteria which must be considered when selecting the lubricant for a particular application. The viscosity requirements for qualifications as multi-grade engine oils are described by the SAE Engine Oil Viscosity Classification-SAE J300. The low temperature (WA) viscosity requirements are determined by ASTM D 5293, Method of Test for Apparent Viscosity of Motor Oils at Low Temperature Using the Cold Cranking Simulator (CCS), and the results are reported in centipoise (cP). The higher temperature (100xc2x0 C.) viscosity is measured according to ASTM D445, Method of Test for kinematic Viscosity of Transparent and Opaque Liquids, and the results are reported incentistokes (cSt). Table 1 below outlines the high and low temperature requirements for the recognized SAE grades for engine oils.
The SAE J300 viscosity grade definitions end at SAE 60 but the scale may be extrapolated in a simple linear manner using the following correlation, which is used in this specification in reference to viscosity grades beyond J300:
In a similar manner, SAE J306c describes the viscometric qualifications for axle and manual transmission lubricants. High temperature (100xc2x0 C.) viscosity measurements are performed according to ASTM D445. The low temperature viscosity values are determined according to ASTM D2983, Method of Test for Apparent Viscosity at Low Temperature Using the Brookfield Viscometer and these results are reported in centipoise (cP). Table 2 summarizes the high and low temperature requirements for qualification of axle and manual transmission lubricants.
In addition to the viscosity temperature relationship, other properties are, of course, required for an engine oil including resistance to oxidation under the high temperatures encountered in the engine, resistance to hydrolysis in the presence of the water produced as a combustion product (which may enter the lubricating circulation system as a result of ring blow-by) and since the finished oil is a combination of basestock together with additives, these properties should be achieved in the final, finished lubricant so that it possesses the desired balance of properties over its useful life
In recent years, considerable attention has been given to the tribological behavior of lubricants under conditions of high shear rate and high pressure. At high shear rates, as in a lubrication contact zone, considerable shear thinning may occur, which results in a decrease in the thickness of the lubricant film separating the relatively moving surfaces with the possibility that inadequate film thickness may be maintained under these conditions. As a counter to this tendency, it would be desirable to provide lubricant compositions which can function effectively under high temperature conditions and which possess good Theological properties to provide adequate. film thickness and wear protection by resisting shear thinning under conditions of high temperature and high shear rate as well as high contact pressure.
As noted above, various combinations of additives with lubricants have been used in the past for the improvement of lubricant properties and in particular, the use of polymeric materials for altering the viscosity or viscosity index of base stocks of mineral and synthetic origin has been well known for a number of years. Polymeric thickeners which are commonly used in the production of multi-grade lubricants typically include hydrogenated styrene-isoprene block copolymers, rubbers based on ethylene and propylene (OCP), polymers produced by polymerization of esters of the acrylate or methacrylate series, polyisobutylene and the like. These polymeric thickeners are added to bring the viscosity of the base fluid up to the level required for the desired grade (high temperature specification) and possibly to increase the viscosity index of the fluid, allowing for the production of multi-grade oils.
The use of high molecular weight thickeners and VI improvers in the production of multi-grade lubricants has, however, some serious drawbacks. First, these improvements are more sensitive to oxidation than the basestocks in which they are used, which may result in a progressive loss of viscosity index and thickening power with use and frequently in the formation of unwanted deposits. In addition, these materials tend to be sensitive to high shear rates and stresses as well as to a high degree of temporary shear the result of which is that temporary or permanent viscosity losses, or reduction of film thickness in bearings may occur. Temporary viscosity losses occurring from shear forces are the result of the non-Newtonian viscometrics associated with the solutions of high molecular weight polymers. As the polymer chains align with the shear field under high shear rates, a decrease in viscosity occurs, reducing film thickness and the wear protection associated with the elastohydrodynamic film. By contrast, Newtonian fluids maintain their viscosity regardless of shear rate. From the point of view of lubricant performance at high temperatures and under the influence of a shear rate condition, it would be desirable to maintain Newtonian rheological properties for the lubricant.
U.S. Pat. No. 4,956,122 (Watts/Uniroyal) discloses lubricating compositions based on combination of low and high molecular weight PAOs which are stated to provide high viscosity index coupled with improved resistance to oxidative degradation and resistance to viscosity losses caused by permanent or temporary shear conditions. According to the invention description in this patent, the lubricating composition comprises a high viscosity PAO or other synthetic hydrocarbon together with a low viscosity mineral derived oil or PAO or other synthetic hydrocarbon such as alkyl benzene. Optionally, a low viscosity ester and an additive package may be included in the lubricants. While the combination of PAO components of varying molecular weight has been effective in a variety of different applications, further improvements in reducing shear thinning characteristics would be desirable, particularly with increasing demands on engine oil performance. Under modern engine manufacturing trends, engines are operating at higher temperatures and as bearing loadings increase as a result of increased specific power output (kW/I), shear thinning conditions are greatly aggravated.
A new type of PAO lubricant was introduced by U.S. Pat. Nos. 4,827,064 and 4,827,073 (Wu). These PAO materials, which are produced by the use of a reduced valence state chromium catalyst, are olefin oligomers or polymers which are characterized by very high viscosity indices which give them very desirable properties to be useful as lubricant basestocks and, with higher viscosity grades; as VI improvers. They are referred to as High Viscosity Index PAOs or HVI-PAOs. The relatively low molecular weight HVI-PAO materials were found to be useful as lubricant basestocks whereas the higher viscosity PAOs, typically with viscosities of 100 cSt or more, e.g. in the range of 100 to 1,000 cSt, were found to be very effective as viscosity index improvers for conventional PAOs and other synthetic and mineral oil derived basestocks.
We have now found that it is possible to use the HVI-PAO oligomers in combination with oils or mineral origin as well as PAO and other synthetic basestocks in combination with high molecular weight polymers such as viscosity modifiers and VI improvers to produce lubricants which are characterized by viscosity thickening properties. Under high shear rate conditions, as in a highly loaded lubrication contact zone, the good viscoelastic properties of the HVI-PAO component produces unexpectedly high film thickness. The improved film thickness provides an unexpected degree of wear protection, resisting shear thinning under conditions where high molecular weight polymers lose some or all of their thickening power. Under low-shear or no-shear conditions, as in low pressure oil circulating systems, the high molecular weight polymer which is used in addition to the low molecular weight viscoelastic polymer, provides enhanced bulk oil viscosity due to its thickening properties under conditions where the low molecular weight polymers have little or no thickening power. Multi-grade and widely cross-graded oils can therefore be produced with a combination of good performance properties which are maintained under varying conditions, but are especially notable under conditions of high temperature high shear rate where they provide unexpectedly good wear protection.
The high performance liquid lubricants of the present invention comprise a first polymer and a second polymer of differing molecular weights dissolved in a liquid lubricant basestock of low viscosity. The first polymer, which is of lower molecular weight than the second polymer, possesses high viscoelastic properties as indicated by its unexpectedly high first normal stress difference. This polymer component in the lubricant provides unexpectedly high film thickness and unexpectedly good wear protection under conditions where high molecular weight polymers lose some or all of their thickening power, for example, at high shear rates in lubrication contact zones. The second polymer, which has a higher molecular weight than the first polymer, is characterized by viscosity thickening properties when blended with the liquid basestock used in the lubricant, which may be either mineral-oil derived or synthetic, preferably a PAO.
In preferred compositions of this type, the basestock is typically a wholly synthetic base oil which may be a single PAO or blend of PAOs which provides the designed viscosity in the final blend, together with the other components including the highly viscoelastic polymer which is preferably one of the HVI-PAO olefin polymers referred to above. The highly viscoelastic component will have a viscosity which is greater than that of the PAO basestock but less than that of the higher molecular weight polymer which is typically one of the polymeric thickeners such as the hydrogenated styrene-isoprene block copolymers, ethylene/propylene rubbers, polyisobutylenes or similar materials referred to above. This polymeric component will typically have a molecular weight in the range from 10,000 to 1,000,000, more usually of at least 100,000.
The use of the highly viscoelastic low molecular weight polymer enables the production of very widely cross-graded engine oils, especially oils with a low temperature grading of 0W or better. Oils with cross gradings of 0W20, 0W30, 0W40 or even more widely cross graded, for example 0W70 or higher may be achieved. Engine oils, cross graded such as 0W70 and 25W70, may achieve excellent wear performance even under conditions of high levels of fuel dilution, indicating that the use of the low molecular weight highly viscoelastic component in combination with the high molecular weight polymer component is capable of countering the deleterious oil film thinning effects of fuel dilution on low viscosity base oils. Another particular achievement of this invention is in formulating very low viscosity highly fuel efficient oils with a 0W low temperature rating, which have a cross-grading of 0W-20 or wider, such as 0W-30, which are capable of passing the ASTM Sequence VE wear test, in which high levels of fuel, water, and blow-by contaminants accumulate in the oil during the 12-day, low-temperature test. Although it has previously been possible to pass the high-temperature Sequence III E wear test with a very low-viscosity 0W-20 or 0W-30 oil, passing the very demanding Sequence V E test had so far been highly elusive.